1. Field of the Invention
The present invention relates to a motor drive PWM rectifier for converting three-phase AC power to DC power by controlling switching devices using a PWM signal (pulse-width modulation signal).
2. Description of the Related Art
In a motor control apparatus for driving a machine tool, industrial machine, robot or the like, a motor drive converter is used that converts AC commercial power to DC power and that supplies the DC power to an inverter for driving a motor.
A diode rectifier is an example of such a converter. While the diode rectifier has the advantage of being inexpensive, the disadvantage is that power supply harmonics and reactive power increase.
In view of this, the application of rectifiers using pulse width modulation (PWM) (hereinafter referred to as “PWM rectifiers”) has been increasing in recent years due to the need to reduce power supply harmonics and reactive power.
FIG. 9 is a diagram showing the configuration of a conventional PWM rectifier. In the PWM rectifier 100, a main circuit 10 includes transistors 12 to 17, diodes 18 to 23, and a smoothing capacitor 24 connected as shown. A three-phase AC power supply 30 is connected via an AC reactor 26 to the input side of the main circuit 10, and a load 32 such as a PWM inverter is connected to the output side.
An adder 36 outputs a difference (voltage difference) of the output voltage of the PWM rectifier 100, i.e., the voltage across the smoothing capacitor 24, from a voltage command. A voltage controller 34 takes as inputs the voltage difference supplied from the adder 36 and the voltage from the three-phase power supply 30, and outputs a current command. An adder 38 outputs a difference (current difference) of the current detected by a current detector 28 provided at the AC input side of the PWM rectifier 100 from the current command. A current controller 140 compares a PWM voltage command, created based on the current difference, with a constant-amplitude, constant-frequency PWM carrier (pulse-width modulation carrier) and, based on the result of the comparison, outputs a PWM signal (pulse-width modulation signal) for controlling the transistors 12 to 17.
FIG. 10 is a diagram for explaining a three-phase modulation scheme used in the conventional PWM rectifier. In FIG. 10, PWM voltage commands for R phase, S phase, and T phase in the three-phase modulation scheme are indicated by solid lines, and the PWM carrier to be compared with them is indicated by dashed lines. In the current controller 140, the PWM voltage command for each phase is compared with the PWM carrier having a triangular waveform, and when the PWM voltage command is larger than the PWM carrier, an associated one of the upper transistors 12, 14, and 16 in FIG. 9 is turned on and an associated one of the lower transistors 13, 15, and 17 is turned off; on the other hand, when the PWM voltage command is smaller than the PWM carrier, the associated one of the lower transistors 13, 15, and 17 in FIG. 9 is turned on and the associated one of the upper transistors 12, 14, and 16 is turned off. As shown in FIG. 10, as the value of the PWM voltage command for each phase varies, the ON period of each transistor connected to that phase varies; i.e., as the value of the PWM voltage command approaches the maximum value of the PWM carrier, the ON period of the upper transistor connected to that phase increases, and as it approaches the minimum value, the ON period of the lower transistor connected to that phase increases.
In the PWM rectifier, switching losses increase because high-speed switching is performed by the switching devices as described above. Accordingly, the PWM rectifier has the problem that, compared with the traditional diode rectifier, losses in the apparatus as a whole increase and the size of the apparatus also increases.
To solve this problem, the prior art has used a technique that decreases the PWM frequency in regions where the amplitude of the AC input current is large. This method is effective in reducing the switching losses (heating) of the switching devices and suppressing the increase in the apparatus size. However, the prior art method has had the problem that the response of the controller degrades because the feedback sampling period is increased as the PWM frequency decreases.
In view of this, there is proposed, for example, in Japanese Unexamined Patent Publication No. 2010-200412, a PWM rectifier that can reduce switching losses without incurring degradation of controllability by using a two-phase modulation scheme which seeks to decrease the number of switching operations by setting and holding one of three-phase PWM voltage commands to a level equivalent to the maximum or minimum value of the PWM carrier in a large current region where switching losses increase.
On the other hand, there is proposed, for example, in Japanese Unexamined Patent Publication No. 2004-048885, a PWM inverter that uses a technique that selects a three-phase modulation scheme or a two-phase modulation scheme, depending on whether precedence should to be given to the accuracy of current control or the suppression of heating, when outputting an AC voltage by converting a DC voltage using a PWM signal.
There is also proposed, for example, in Japanese Unexamined Patent Publication No. H08-023698, a PWM inverter that uses a technique that switches the modulation scheme from the three-phase modulation scheme to the two-phase modulation scheme to reduce the distortion of actual current waveform when the motor is in regenerative mode or to suppress transient fluctuations in current and torque when the motor is switched from regenerative mode to powering mode.
According to the above two-phase modulation scheme, the switching losses (heating) of the switching devices can be reduced, but the proportion of ripple components (harmonic components) relative to the fundamental component of the AC input current increases, since the number of switching operations is smaller than in the case of the three-phase modulation scheme. That is, when it is assumed that the fundamental component of the AC input current is the same between the two-phase modulation scheme and the three-phase modulation scheme, there arises the problem that the peak value of the AC current becomes larger in the case of the two-phase modulation scheme.
Generally, in a PWM rectifier, in order to prevent the switching devices from being operated above their maximum rated current, a protective function is provided that monitors the AC input current in real time and that, when the input current value exceeds a predetermined value, causes the PWM operation to stop by issuing an alarm or by forcefully stopping the switching operation and limiting the current by hardware means.
However, since the proportion of ripple components relative to the fundamental component of the AC input current increases in the case of the two-phase modulation scheme, as described above, the two-phase modulation scheme has the problem that the protective function is activated earlier than in the case of the three-phase modulation scheme. FIG. 11a is a diagram for explaining a current ripple that occurs at the AC input side of the conventional PWM rectifier in the case of the three-phase modulation scheme. FIG. 11b is a diagram for explaining a current ripple that occurs at the AC input side of the conventional PWM rectifier in the case of the two-phase modulation scheme. In FIGS. 11a and 11b, the fundamental component of the AC input current plus the ripple component is indicated by solid lines, and the fundamental component of the AC input current is indicated by dashed lines. As compared with the current ripple in the case of the three-phase modulation scheme shown in FIG. 11a, the magnitude of the current ripple relative to the fundamental component of the AC input current increases in the case of the two-phase modulation scheme, as shown in FIG. 11b, because the number of switching operations in the region where the current peaks is smaller than in the case of the three-phase modulation scheme. When it is assumed that the activation level of protection function is set as shown in FIGS. 11a and 11b, and that the fundamental component of the AC input current is the same between the two-phase modulation scheme and the three-phase modulation scheme, since the magnitude of the current ripple becomes larger in the two-phase modulation scheme than in the three-phase modulation scheme, the peak value of the AC current increases and, as a result, the protective function becomes easier to activate.
Therefore, according to the method that uses as the modulation scheme for the PWM rectifier the three-phase modulation scheme in the region where the AC input current is small and the two-phase modulation scheme in the region where the AC input current is large, there arises a need to operate the PWM rectifier by reducing its output so as not to activate the protective function; this has led to the problem that, compared with the method that uses the three-phase modulation method at all times regardless of the magnitude of the AC input current, the currents in the switching devices cannot be effectively used, and thus the maximum output of the PWM rectifier drops.